The modern pharmacopeia is a glorious thing—drugs have helped many people with depression and anxiety, not to mention cancer and other life-threatening diseases. But what goes down your throat must eventually come out, er, elsewhere—especially since drugs are designed specifically so they won’t break down on the pharmacy shelves or in a patient’s body. Starting in the 1990s, with the advent of super-sensitive chemical-detection technology, scientists began discovering that a lot of this medical chemistry is surviving wastewater treatment plants and flowing into waterways. That, in turn, raised a basic and troubling question: What’s it doing to the fish?

Typically, the impact of industrial chemistry on wildlife is tested using fairly basic measures of toxicity. If it doesn’t kill animals outright or prevent them from reproducing—as DDT did by causing birds to lay thin-shelled eggs—it’s not considered a clear and present threat. But with more and more psychotropic drugs like antidepressants and anti-anxiety drugs flowing from factories, into consumers and out into the wild, environmental scientists have begun to worry that the meds may be affecting animal behavior too. In a paper published this week in the journal Science, Swedish researchers report that they put that question to the test—and they came up with some troubling answers.

The scientists, all affiliated with Sweden’s Umeå University, began by testing perch, a species of schooling fish, living downstream from a wastewater treatment plant. The investigators were specifically looking for traces of the anti-anxiety drug oxazepam, which has been observed in waste water before. They found the drug in the water and, when they examined muscle samples of the fish, saw it there too—but in six times the concentration it is in the river, suggesting that it builds up in the animals’ bodies over time. Next, they started afresh with a school of perch they hatched from eggs and, when they had grown, put them through a battery of tests to measure behavioral qualities thought important for perch survival, such as schooling and danger avoidance. Then they split the school into three groups: the first would live in a tank with clean water, the second would swim in water with a same concentration of oxazepam on the order of what was found in the river, and the third would get water with 500 times the river’s concentration.

Once the fish had been in their respective treatments for a week, the researchers ran the behavioral tests again. To see whether the fish sought the company of others, they released each perch into the central compartment of a three-part tank, which had three other perch on one side and an empty compartment on the other, and recorded where and how the central fish swam for about ten minutes. Then, to measure the fish’s boldness, they put each perch into a small, dark box for 5 minutes in another tank, opened a door in the box’s side, and watched to see how long it took the fish to venture out of its haven. Finally, they put each fish in a tank stocked with 20 zooplankton, the tiny creatures perch like to eat, and recorded how long it took them to snap up their prey.

The clean-water fish were unchanged after their time in their pristine tank, performing the same way as they had a week before. The fish exposed to a low level of oxazepam spent significantly less time near other fish, darted around more frequently, and gobbled up their food much more quickly than they had a week prior. The effects were even more pronounced in the high-exposure fish, particularly in the boldness test. Leaving the dark box requires more chutzpah than most perch can muster; not a single clean-water fish or low-dose oxazepam fish would do it. But a remarkable 23 of the 24 high-dose fish came out. “It’s an extreme effect,” says Tomas Brodin, assistant professor of ecology and an author of the paper. “They get fearless.” Brodin suspects that the low-dose fish’s more hyperactive, less social behavior is a milder version of the high-dose fish’s boldness, all implying a reduction of caution. “That’s bad, if you’re a little schooling fish,” he adds.

The Umeå researchers do not suggest oxazepam or any psychotropic operates exactly the same way on fish that it does in humans. At the same time, the drug works through a particular cellular receptor that humans and perch—and many other species—share, so there is a potential for overlaps. That has Bryan Brooks, a professor of environmental science at Baylor University with long experience studying pharmaceuticals in the environment, arguing for a new testing standard for drug toxicity. “We should be thinking about behavior in a compound that affects behavior,” he says.

If groups like the US Environmental Protection Agency are ever to draft regulations to improve the control of drug release into the environment, they will need just that kind of testing data to determine which drugs need the most attention and how to get rid of them. That could lead to demand for new filtering technologies—and those systems could be put straight to work. “The good thing is that we have the effluent in the pipe,” says environmental scientist Jerker Fick, another author of the paper. “If someone comes up with a good solution to remove [the drugs], we have a place for it.”